1. Field of the Invention
The present invention relates to a semiconductor device utilizing a semiconductor thin film having a crystal structure and a fabrication method thereof. Particularly, the present invention relates to a semiconductor device using a thin film transistor (hereinafter, abbreviated as TFT) having an inverted stagger structure.
Further, "semiconductor device" in the specification designates all of devices operated by utilizing semiconductor properties. Accordingly, all of TFTs, AMLCDs (Active Matrix Type Liquid Crystal Display Device) and electronic devices described in the specification are included in the category of semiconductor devices.
2. Description of Related Art
Conventionally, there has been utilized TFT as a switching element of an active matrix type liquid crystal display device (hereinafter, abbreviated as AMLCD). Currently, a product constituting a circuit by TFT utilizing an amorphous silicon film as an active layer is predominant in the market. As structure of TFT, an inverted stagger structure the fabrication steps of which are simple has been adopted frequently.
However, high function formation of AMLCD has been progressed year by year and operational function (particularly, operational speed) requested to TFT tends to become severe. Therefore, it is difficult to provide an element having sufficient function with operational speed of TFT using an amorphous silicon film.
Hence, TFT utilizing a polycrystal silicon film (polysilicon film) has been spotlighted in place of an amorphous silicon film and development of TFT with a polycrystal silicon film as an active layer has been progressed with significant vigor. At present, product formation thereof has also been carried out partially.
Many presentations have already been carried out in respect of a structure of an inverted stagger type TFT utilizing a polycrystal silicon film as an active layer. For example, there have been reports of "Fabrication of Low-Temperature Bottom-Gate Poly-Si TFTs on Large-Area Substrate by Linear-Beam Excimer Laser Crystallization and Ion doping Method: H. Hayashi et. al., IEDM 95, pp. 829-832, 1995", the disclosure of which is herein incorporated by reference, and so on.
According to the report, an explanation has been given of a typical example of an inverted stagger structure utilizing a polycrystal silicon film (FIG. 4), however, the inverted stagger structure of such a structure (so-called channel stop type) poses various problems.
First, a total of an active layer is extremely thin to about 50 nm and accordingly, impact ionization is caused at a junction between a channel forming region and a drain region and deteriorating phenomena of hot carrier injection and the like significantly emerge. Therefore, there causes a necessity of forming a large LDD region (Light Doped Drain Region).
Further, controllability of the LDD region for these is the most important problem. In an LDD region, control of a concentration of impurities and a length of the region is very delicate and particularly, control of length becomes problematic. At current state, a system for prescribing a length of an LDD region by a mask pattern is adopted, however, when miniaturization is promoted, slight patterning error causes a significant difference in properties of TFT.
A variation in sheet resistance of an LDD region caused by a variation in a film thickness of an active layer poses a serious problem. Further, a variation in a taper angle or the like of a gate electrode can be a factor causing a variation in the effect of an LDD region.
Further, a patterning step is needed to form an LDD region which gives rise to an increase in fabrication steps as it is and a deterioration in throughput. According to an inverted stagger structure described in the report mentioned above, it is anticipated that 6 sheets of masks at minimum (until formation of source and drain electrodes) are necessary.
As mentioned above, according to an inverted stagger structure of a channel stop type, LDD regions must be formed on both sides of a channel forming region and in a plane in the horizontal direction and it is very difficult to form a reproducible LDD region.
Further, the conventional AMLCD is provided with a structure in which a storage capacitance is installed to each pixel to compensate for leakage of electric charge held in a liquid crystal layer.